Field of the Invention
The invention relates to a honeycomb body of wound, stacked or otherwise stratified layers formed of at least one metal sheet, in which the layers form a plurality of channels which are substantially parallel to one another and through which a fluid can flow; the layers are electrically connected among one another in such a way that there is at least one path for carrying an electric current through the honeycomb body for the purpose of heating the same.
There are manifold uses for honeycomb bodies as carrier bodies for catalysts, intended for the catalytic conversion of reactable components of fluids. One particular field in which honeycomb bodies with catalysts are used is the catalytic cleaning of exhaust gases from internal combustion engines, particularly for motor vehicle engines. To that end, catalytically coated honeycomb bodies are installed in the engine exhaust systems, and the exhaust gases produced during engine operation flow through them.
The honeycomb bodies are manufactured from ceramic masses or from metal sheets. To form a metal honeycomb body, layers of metal sheets, some of which may be corrugated, folded or similarly structured, are stratified, stacked, spirally wound or otherwise intertwined. Possible ways of doing this are described in European Patents 0 223 058 B1 (corresp. to U.S. Pat. No. 4,824,011), 0 245 737 B1 (corresp. to U.S. Pat. Nos. 4,832,998 and 4,923,109), and 0 245 738 B1 (corresp. to U.S. Pat. Nos. 4,803,189 and 4,946,822); U.S. Pat. Nos. 4,753,918 and 4,822,766; Published International Applications WO 89/10470 A1, WO 89/10471 A1, and WO 90/03220 A1, and German Utility Model DE 89 08 738 U1.
Catalysts for converting reactable components of a fluid flowing around them are not catalytically effective until above certain threshold temperatures, the so-called critical temperatures. These are specific to the particular catalyst and to the reaction to be catalytically supported. In the case of catalysts for converting engine exhaust gas pollutants, the critical temperatures are typically several hundred degrees Celsius. For a catalyst to become active, it must accordingly be heated to a temperature above the critical temperature. In the exhaust system of a motor vehicle, this is effected as a rule by the exhaust gas flowing through catalyst-coated conduits, but then the catalytic action lags behind the starting of the engine.
To reduce this lag, electrical preheating of the catalyst, or of the honeycomb body carrying it, has been proposed heretofore. Instructions along these lines can be found in the above-noted WO 89/10470 A1, WO 89/10471 A1, and DE 89 08 738 U1. The layers of sheet metal that form the honeycomb body are electrically connected among one another in such a way that at least one path for carrying an electric current through the honeycomb body is available. The honeycomb body is also provided with power supply lines, to which a source of emf, an electric current source, such as a motor vehicle battery, is connected via suitable switching devices.
A honeycomb body of the usual size and structural type for use in the exhaust system of a motor vehicle requires a heating capacity of from several hundred watts to over 4 kW, in order to heat it up sufficiently quickly. Accordingly, an on-board electric system with a voltage of, say, 12 V, which is conventional in passenger cars, must furnish currents of up to more than 400 A for heating the honeycomb body. The problematic aspect here is the fact that a typical prior-art honeycomb body has an electrical resistance of at most several thousandths of an ohm. From a 12 V voltage source, such as a honeycomb body would draw currents of more than 1000 A. That, of course, would strain a typical on-board motor vehicle electrical system, in particular its battery, to a hardly allowable extent.
Provisions for increasing the electrical resistance of honeycomb bodies are already known as well. In the above-noted publications WO 89/10470 A1 and WO 89/10471 A1, electrically heatable honeycomb bodies are subdivided by gaps and/or electrically insulating partitions between the layers, in such a way as to produce at least one electric current path through the honeycomb body with an electrical resistance at a level that, with a voltage on the order of the usual voltage in an on-board motor vehicle electrical system, furnishes an allowable level of current through the honeycomb body and heat development of appropriate capacity in the honeycomb body.
In a further development, it has been proposed to provide two monoliths in series, i.e. instead of a single honeycomb body with a catalytic coating, two honeycomb bodies are used and the smaller, electrically heatable honeycomb body precedes the "main" catalyst carrier body. The larger honeycomb body need not be electrically heated. Both honeycomb bodies have essentially the same diameters, but the smaller honeycomb body is substantially shorter than the larger one. By reason of dimensions alone, a higher electrical resistance can be achieved with the smaller honeycomb body than with the larger one. Hence at a limited load on the voltage source, the smaller honeycomb body can be relatively quickly brought to a temperature above its critical temperature, whereupon the catalytically aided reaction ensues in the exhaust gas that flows first through the smaller honeycomb body and then through the larger honeycomb body. Due to the fact that the reaction is exothermic, this contributes to the further, accelerated heating of the larger honeycomb body, which finally takes on the majority of the task of catalytic conversion.
According to DE 89 08 738 U1, the sheet-metal layers that are provided for producing the honeycomb body may be provided with openings. By forming openings in the layers, a specific resistance of the sheet metal can be increased, making a honeycomb body with relatively high electrical resistance possible. However, this provision, like all other provisions aimed at reducing the thickness of the sheet metal used to make up a honeycomb body, becomes ineffective with increasing size of the honeycomb body. Enlarging a honeycomb body of a known type, while maintaining the ratios of the various dimensions to one another, always involves a reduction of its electrical resistance. In that, the teachings of the prior art must be combined with limitations in the dimensions of the honeycomb body. In turn, this means that in the prior art, in systems beyond certain dimensions, it is necessary to revert to two-piece honeycomb bodies, with all of the attendant problems.